1. Field of the Invention
The present invention relates generally to the fields of power and power supplies. More particularly, it concerns methods and apparatuses for providing a controllable and variable high-voltage, logic-controlled, resonant switching power supply. Even more particularly, it concerns methods and apparatuses for providing power exhibiting low electromagnetic interference (EMI); thus, the invention may be used for driving, for example, avalanche photo diodes (APDs), which typically require high voltage but are sensitive to excess EMI.
2. Description of Related Art
The ability to provide power exhibiting low EMI is important in a wide variety of disciplines. For instance, sensor equipment such as avalanche photo diodes, medical equipment, communication equipment, and a variety of other general electronic applications may greatly benefit from utilizing low EMI power.
EMI can negatively impact communications systems and interfere with the proper operation of sensitive electronic equipment. For example, in medical applications, EMI may cause false readings and, in extreme cases, may disrupt the proper operation of life-sustaining electronic devices such as cardiac pacemakers. In the case of sensor equipment such as APDs, EMI may contribute to noise, which may disrupt readings by interfering with the measurement of the signal of interest.
As is known in the art, conventional transformers and switched-mode power supplies (SMPS) may contribute to EMI. SMPS typically operate at switching frequencies (and harmonics of those frequencies) that fall well within the frequency bands commonly allocated for communications and other devices, such as high-frequency (HF), very-high frequency (VHF), and ultra-high-frequency (UHF) bands. Consequently, their leakage signals represent a potential source of interference to consumer, commercial, and military communications services worldwide, as well as other analog and digital electronic systems such as photodiode systems, radars, and medical electronic systems that may be hindered by the interference.
Much of the noise generated by SMPS is related to the switching process itself. In its open position, an ideal switch provides infinite resistance to the flow of electrical current. In its closed position, the ideal switch offers zero resistance, allowing current flow with negligible drop in voltage. An ideal switch would control the conduction of high-speed pulsed waveforms without adding transient events or causing voltage spikes. However, real switches cannot change states instantaneously. Instead, they require transition periods known as rise and fall times. These transition periods impose distortion on the harmonic components of high-frequency SMPS waveforms. The switching process produces voltage spikes resulting in EMI and radio frequency interference (RFI) that can reach well past 100 MHz. The voltage spikes are typically caused by short-duration charging and discharging of parasitic capacitances in the power-supply circuitry.
In extreme cases where transformer or SMPS EMI levels are high, at least some of the interference may be controlled with a magnetic shielding enclosure. The enclosure surrounds the transformer, captures stray magnetic flux or radiation, and channels the unwanted energy to a ground plane. Unfortunately, such enclosures are typically bulky and add cost, weight, and manufacturing complexity to a design.
Aspects of the present disclosure may be used in conjunction with conventional apparatuses and methods relating to power supplies in general, and more particularly to switching power supplies. Below, several U.S. patents that are representative of such conventional technology are briefly discussed. Although each of these references has shown at least a degree of success in its respective application, room for significant improvement remains.
U.S. Pat. No. 5,448,465, which is hereby incorporated by reference, involves a switching power supply that supplies a regulated output voltage and suppresses spike voltage and spike current generated by a switching action of transformers. It also suppresses switching frequency drift due to load variation. A reduction of noise interference to electronic equipment and power loss in the switching power supply itself results. A series connection of a primary winding of a first transformer and a first switching means, which repeats an on/off action is connected across a DC power source. A series connection of a second switching means, which repeats an on/off action alternately with the first switching means, and a first capacitor is connected in parallel with the primary winding of the first transformer. A series connection of a primary winding of the second transformer and a second capacitor is connected in parallel with the second switching means. Regulated DC outputs are taken out from each secondary winding of the transformers through rectifiers/filters. A control circuit supplies on/off signals, which on/off-ratios are varied according to one of the regulated DC outputs, to the first and the second switching means.
U.S. Pat. No. 5,671,128, which is hereby incorporated by reference, involves a power supply apparatus in which a series circuit of first and second switching elements not blocking their reverse-directional currents is connected in parallel to a series circuit of a DC power source and a capacitor. A primary winding of a transformer is connected between a junction point of the DC power source and capacitor and a junction point of the switching elements. A secondary winding of the transformer is connected to a load circuit, and a controller is provided for controlling ON and OFF operation of the switching elements to cause a switching frequency of the switching elements to be set higher than a resonance frequency of the capacitor and an inductance of the transformer and to cause a voltage across the capacitor to increase.
U.S. Pat. No. 5,715,155, which is hereby incorporated by reference, involves a resonant switching power supply circuit that includes positive and negative DC input terminals. First and second switching elements are connected in series between the DC input terminals. Each switching element has a control input for receiving a control signal controlling a duty cycle of the switching element. A first capacitor is connected between the DC input terminals. Second and third capacitors are connected in series between the DC input terminals. A first pair of reverse biased rectifying elements in series is connected between the DC input terminals. Each rectifying element of the first pair is connected in parallel with a respective one of the switch elements. A second pair of reverse biased rectifying elements in series is connected between the DC input terminals. Each rectifying element of the second pair is connected in parallel with a respective one of the second and third capacitors. The circuit also includes a transformer including a primary winding having opposite winding ends connected between the rectifying elements of the first pair and between the rectifying elements of the second pair respectively, and a secondary winding having opposite winding ends. A pair of output terminals is connected to the winding ends of the secondary winding of the transformer respectively to supply power. The resonant switching power supply circuit can be used as a power inverter or converter.
U.S. Pat. No. 6,018,467, which is hereby incorporated by reference, involves a resonant mode power supply. It includes a DC voltage source and switching elements for alternatively connecting an oscillating circuit, including a primary winding of a transformer, to the DC voltage source and to ground. A first secondary winding supplies the main output voltage of the power supply, and a second secondary winding provides a control output voltage. An opto-coupler is included and has a light emitter connected to the second secondary winding and a light sensor connected to a controller for controlling the switching of the switching elements. The higher the frequency of the switching, the lower the power being supplied by the power supply. Burst mode stand-by is started by the first secondary winding being shunted to the second secondary winding causing the light emitter to emit a maximum amount of light. The controller increases the switching frequency until it exceeds a predetermined maximum frequency. The controller then stops the switching of the switching elements until the voltage across the light emitter falls below a predetermined value, and the controller then re-starts the switching.
U.S. Pat. No. 6,087,782, which is hereby incorporated by reference, involves a resonant mode power supply. It includes a DC voltage source and switching elements for alternatively connecting an oscillating circuit, including the primary winding of a transformer, to the DC voltage source and to ground. In order to detect faults in the load on a secondary side of the transformer which would cause the resonant mode power supply to attempt to supply and inordinate amount of power, the power on the primary side is detected. And, if this primary power exceeds a predetermined threshold value, the frequency of oscillation is increased to reduce the power. If the fault condition persists, the switching of the switching elements is discontinued.
U.S. Pat. No. 6,130,825, which is hereby incorporated by reference, involves a switching power supply. Primary-side MOS transistors are alternately turned on so that a resonant current flows into the primary winding of a transformer, and an alternate power is transferred to the secondary side. The alternate voltage generated at the secondary winding is applied by the voltage generated at the wound-up secondary winding to the gates of secondary-side MOS transistors such that they are turned on respectively in periods when the polarity of the voltage is positive. Rectified currents flow into a capacitor through a choke coil to perform synchronous rectification. If the voltage of a smoothing capacitor becomes higher than the alternate output voltage when the transformer is inverted, reverse currents flow into the secondary-side MOS transistors. With the counterelectromotive force of the choke coil, the reverse currents flowing when the transformer is inverted are suppressed, and the efficiency of the switching power supply is prevented from decreasing.
U.S. Pat. No. 6,147,881, which is hereby incorporated by reference, involves a resonant switching power supply that has a zero voltage and zero current switch feature that can be operated in a half-bridge or a full-bridge scheme. This enables power consumption to be reduced and electromagnetic radiation to be minimized, and it provides for low cost and convenient manufacture in mass production. The power supply is not influenced by parasitic capacitance and leak inductance.
U.S. Pat. No. 6,157,179, which is hereby incorporated by reference, involves a switched-mode power supply specially suitable for supplying a low output power. The switched-mode power supply periodically first charges a capacitor from an input voltage during a first period of time. It then forms a resonant circuit, including the capacitor and an inductor, to transfer the charge in the capacitor to a load via a rectifier during a second period of time.
U.S. Pat. No. 6,278,620, which is hereby incorporated by reference, involves a switching power-supply circuit comprising: rectifying and smoothing means for generating a rectified and smoothed voltage and outputting the rectified and smoothed voltage as a direct-current input voltage; an insulating converter transformer for transferring a primary-side output to a secondary side; switching means for intermittently passing on the direct-current input voltage to a primary winding of the insulating converter transformer; a primary-side resonance circuit for actuating the switching means in a voltage resonance mode; power-factor improvement means for improving a power factor by generating intermittently a rectified current based on the fed-back switching output voltage; a secondary-side resonance circuit on a secondary side of the insulating converter transformer; direct-current output voltage generation means carrying out a rectification operation in order to generate a secondary-side direct-current output voltage; and constant-voltage control means for executing constant-voltage control on the secondary-side direct-current output voltage.
U.S. Pat. No. 6,288,504, which is hereby incorporated by reference, involves a deflection current/high voltage integration type power supply that has a flyback transformer having a primary winding and a secondary winding. It includes a series circuit of a deflection coil and a first capacitor, connected in series to the primary winding of the flyback transformer; a resonance capacitor connected in parallel to the series circuit of the deflection coil and the first capacitor; a first switching element, connected in parallel with the series circuit of the deflection coil and the first capacitor, to be turned on/turned off by a drive signal so that a high voltage is generated at the secondary winding of the flyback transformer and a deflection current flows in the deflection coil; a parallel connection circuit of a second switching element and a second capacitor, connected in series to the primary winding of the flyback transformer; and a switching control means to control the deflection current flowing in the deflection coil and the high voltage generated at the secondary winding of the flyback transformer by controlling the on-timing and the off-timing of the second switching element in approximate synchronism with the drive signal.
U.S. Pat. No. 6,317,337, which is hereby incorporated by reference, involves a switching power supply circuit that has an insulated converter transformer with a gap for a loose coupling on the secondary side. A parallel resonant capacitor is connected parallel to a secondary winding, and a full-wave rectification circuit produces a secondary-side DC output voltage for increasing a maximum load power. On the primary side, an ordinary full-wave rectification circuit, rather than a voltage doubler rectifying circuit, inputs a rectified and smoothed voltage having a level corresponding to the level of an AC input voltage. A constant-voltage control circuit system for stabilizing a secondary output voltage varies a switching frequency depending on the level of the secondary output voltage to perform composite control over a resonant impedance of a primary parallel resonant circuit and a conduction angle of the switching element.
Although each of the above references exhibit advantageous qualities for its particular application, room for significant improvement remains due to the inability of conventional systems to provide a controllable and variable high voltage power supply from a small number of devices while, at the same time, producing low EMI. Relatedly, conventional systems lack the efficiency, small size, and low EMI with inherent “soft-start” exhibited by embodiments of the present invention described below. Moreover, conventional systems do not allow for operation with a single inductor, no transformer, and one power switch, in contrast to certain specific embodiments described below. Finally, conventional systems typically obtain resonance with a capacitor in series with an inductor, whereas specific embodiments herein are able to obtain resonance with parasitic capacitances in parallel with an inductor.
The referenced shortcomings of the prior art are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning low-EMI power supplies. Other noteworthy problems may also exist; however, those mentioned here are sufficient to demonstrate that methodology appearing in the art have not been altogether satisfactory.